In some situations you want to share reverse routers between sub projects that are not dependent on each other.

For example, you might have a web sub project, and an api sub project. These sub projects may have no dependence on each other, except that the web project wants to render links to the api project (for making AJAX calls), while the api project wants to render links to the web (rendering the web link for a resource in JSON). In this situation, it would be convenient to use the reverse router, but since these projects don’t depend on each other, you can’t.

Play’s routes compiler offers a feature that allows a common dependency to generate the reverse routers for projects that depend on it so that the reverse routers can be shared between those projects. This is configured using the aggregateReverseRoutes sbt configuration item, like this:

In this setup, the reverse routers for api and web will be generated as part of the common project. Meanwhile, the forwards routers for api and web will still generate forwards routers, but not reverse routers, because their reverse routers have already been generated in the common project which they depend on, so they don’t need to generate them.

Note that the common project has a type of Project explicitly declared. This is because there is a recursive reference between it and the api and web projects, through the dependsOn method and aggregateReverseRoutes setting, so the Scala type checker needs an explicit type somewhere in the chain of recursion.